The invention relates to a medical examination apparatus, the magnetic field of which can be applied in operation to an organism to be examiner, that is, notably a human or animal.
In examination apparatus of this kind there is customarily provided a space for receiving a patient. The space is typically situated in the axial interior room of an outer coil. When current is applied to the coil, it produces a magnetic field that penetrates the patient who is arranged inside the coil and which is capable of creating a variety of effects that are used for imaging methods. In particular a plurality of magnet coils may be provided so as to generate first a static magnetic field, secondly an alternating field that acts as a gradient field for the acquisition of position information, and finally an RF magnetic alternating field so as to cause resonance. Overall a plurality of magnetic fields are superposed. The RF magnetic alternating field is capable of exciting magnetic resonance that can be used for the re-orientation of nuclear spins. Such a spin excitation can be evaluated as a spatially resolved image of the organism examined, because it is contained completely in the inhomogeneous magnetic field and hence the resonance frequency varies in any position of the inhomogeneous field.
One or more magnets may be embedded in a cryostat so as to enable a temperature below the critical temperature for superconductivity. In order to generate an adequately strong magnetic field, it is necessary to keep the field generating coil in the superconducting state. The cryostat that is filled with liquid helium of a temperature of 4.2xc2x0 K is provided for this purpose.
The strong electromagnetic force (Lorenz force), being in the range of 100 kN, causes deformations of the coil that generates the gradient field. This results in a vibration that is transferred to the cryostat in which the gradient coil is mounted. This gives rise to a substantial amount of annoying noise, that is, in conjunction with switching operations for manipulating the gradient field whereby eddy currents are induced in the metal parts of the cryostat, and also in conjunctions with other sources of noise. The noise on the one hand is transferred as structure-borne noise directly to the floor, via parts of the apparatus such as the cryostats and its supports on the floor, and on the other hand as airborne noise to the environment. Consequently, it is also transferred in the room in which the examination apparatus is installed. As a result, the partitioning of the room, notably the floor, also acts as a conductor for the noise. Therefore, neighboring rooms and rooms that are situated below the examination apparatus are also exposed to a significant noise load.
It is an object of the invention to improve the aforementioned situation.
The transfer of noise through an air space underneath the examination apparatus to the floor (for example, an intermediate ceiling) on which the examination apparatus is mounted can be significantly reduced by providing a noise-absorbing material in a plane that is situated underneath at least one switchable magnet. In addition to the damping of vibrations within the supports on the floor, a second form of damping is thus realized, which second form not only reduces the transfer of vibrations via fixed components but also dampens the noise that is transferred through the air. The noise damping material can be used for reducing the acoustic load also without vibration damping in the supports.
Preferably, the noise insulation is provided between the supports on the floor, because at least one enclosure that elsewhere envelops the examination apparatus and already provides damping, for example a lining, can thus be dispensed with.
It will be evident that a noise-absorbing material may be provided not only in the lower region but also in other peripheral regions of the examination apparatus, for example, also additionally within a lining.
When the noise-absorbing material is precut, for example, as a mat or a similar element, it can also be introduced into already installed examination apparatus at a later stage.
Further advantages and details of the invention will become apparent from the following description of an embodiment of the invention that is shown in the accompanying drawing.